The present invention relates, in general, to an electrical circuit and method for mitigating the effects of a current increase due to a fault within the circuit. The invention is particularly, but not exclusively, concerned with a current increase across a laser diode due to an electrical short within the circuit.
In the field of integrated circuits, it is known that faults may occur in the structure of an integrated circuit. These structural faults may occur during the manufacturing process or alternatively they may arise during use either as a result of material weaknesses or misuse of the circuit. It is very difficult to locate all faults in an integrated circuit prior to use. Unfortunately, undetected faults may cause the integrated circuit (and any electrical devices that it is coupled to) to fail in their operation.
It is known in the art to design electrical circuits which are xe2x80x98fault tolerantxe2x80x99. A fault tolerant circuit is generally configured so that a failure of strategic components does not result in the complete loss of circuit operation.
Where an integrated circuit includes a laser diode within the circuitry, undetected faults may be especially problematic. For example, if the integrated circuit were shorted to ground as a result of a component of the integrated circuit failing, then a significant increase in current across the laser diode may occur. The current increase may cause light intensity emitted from the laser diode to increase because the light intensity output from the laser diode is proportional to the current drawn. An increase in light intensity output may represent significant danger to a user of the laser diode circuit, the danger arising from inadvertent projection of the laser light into the user""s eye. In this regard, all laser circuits must be stringently tested for compliance with stipulated regulatory requirements, which testing is arduous and therefore expensive. In any event, even if a circuit passes the regulating (safety) tests, there is no guarantee that a fault will not emerge with time with a particular device, which fault could generate a localised current that drives the laser divide output beyond stipulated light intensity outputs and into an unsafe operational state.
There is therefore a need to produce an electrical circuit which mitigates the problem of an increase in current resulting from an undetected or unexpected electrical fault.
According to a first aspect of the present invention, there is provided an electrical circuit containing a first circuit having associated therewith a first track supporting, in use, a first current; and a second circuit drawing, in use, a second current, the second circuit located proximate to the first track, the electrical circuit characterised by an electrical shield providing an electrically isolated enclosure, the electrical shield positioned substantially about the first track and such that the shield inhibits, in use, shorting of the first track to the second circuit to restrict, in use, substantial summing of the first current with the second current.
The shield, in use and under fault conditions, may inhibit establishment of a short circuit supporting flow of a current greater than a predetermined threshold through an electrical component.
The electrical component may be a laser diode, and the second circuit may be a track. Also, the shield may comprise at least one metal layer within an integrated circuit or printed circuit board, the shield further including at least one via.
In a second aspect of the present invention there is provided an electrical device comprising the electrical circuit of the first aspect of the present invention.
In a third aspect of the present invention there is provided an integrated circuit or printed circuit board comprising the electrical circuit of the first aspect of the present invention or the electrical device of the second aspect of the present invention.
In a fourth aspect of the present invention there is provided a method of mitigating effects of a short circuit fault condition within an electrical circuit, the method comprising determining a current sensitive circuit; and providing a ground insulated shield substantially about said current sensitive circuit to prevent, in use, a short circuit fault condition associated with a second electrical circuit from increasing current through the current sensitive circuit.
The invention may also comprise a method of laying out the electrical circuit such that at least one metal layer and at least one via of the non-critical track form a shield around a greater part of the determined critical track.
Advantageously, a fault tolerant integrated circuit may be achieved by the layout of the circuit components and, where the circuit includes a laser diode, a sudden increase in light intensity output by the laser diode (which may damage the eyes of the user) may be avoided. Specifically, a track including the laser diode is identified and insulated by means of a layout of the circuit (metal layers and vias are utilised to form an insulating shield around the laser diode).
The present invention is generally applicable to electrical circuits which require protection from current overload and, whilst being particularly applicable to integrated circuits which include at least one laser diode, can be employed more widely.